The present invention relates to a second harmonic generator which utilizes Cerenkov radiation useful as a source of coherent short-wave light.
Such a second harmonic generator is useful as a light source for recording and/or reproducing in optical information processing systems such as optical disks, laser printers or color printers.
There has heretofore been known technology for generating second harmonics of Cerenkov radiation by forming organic materials having nonlinear characteristics into a device of fiber form and inputting coherent fundamental light thereinto, as described, for example, in Nonlinear Optical Materials-Extended Abstract, 1985, pp. 97-99.
The Cerenkov radiation is composed of second harmonics radiated by a polarization wave having a phase velocity greater than the phase velocity of a medium. However, since the beam shape of the Cerenkov radiation has a cone or an arc shape, it cannot be focused into a good spot.
Furthermore, the generation of second harmonics by Cerenkov radiation from waveguides fabricated on the surface of a substrate of nonlinear crystals such as lithium niobate (LiNbO.sub.3) has been reported in CLEO '87, Technical Digest, pp. 198-199. This method has many advantages in that the secondary light having a wavelength one-half that of the fundamental light can be generated with a high conversion efficiency. In addition, the phase matching of the two light waves having different wavelengths can be achieved relatively easily.
With this method, however, as shown in FIG. 8, since a radiation mode is established from a narrow line-shaped waveguide into the substrate, the light diverges in the direction of radiation while the light is collimated in parallel in the direction at right angles thereto. Therefore, the light cannot be focused to a good spot. Specifically, in the prior art shown in FIG. 8, the substrate of the waveguides consists simply of a block having a flat surface without being devised in particular. Thus, the Cerenkov beam which has passed through the substrate develops a large aberration, and it cannot be focused into a good spot unless a correction measure is employed.
As described above, in the prior art technologies, the countermeasure for the aberration of the Cerenkov beam is insufficient, and thus, it is impossible to focus the Cerenkov beam into a diffraction limited spot. As a result, the Cerenkov beam cannot be used as a light beam for recording and/or reproducing in optical information processing system such as optical disks and the like.